1. Field of the Invention
This invention relates to a shutter time switching device for switching the shutter time of a rotary shutter employed in an exposure control system of a camera and more particularly to preventing occurrence of an error in a rotary shutter time switching device which controls the opening angle of a rotary shutter by means of a stepping motor. The invention further relates to a rotary shutter time switching device which is quickly responsive.
2. Background of the Prior Art
Prior art exposure control systems using a rotary shutter control the shutter opening angle by manual operation of a shutter time setting dial and by adjusting the opening angle of the shutter through a cam, a lever, etc. This arrangement, however, makes it impossible to carry out an aperture-priority automatic exposure control or programmed automatic exposure control.
To permit programmed automatic exposure control, it is conceivable to have the opening angle of the rotary shutter controlled by a stepping motor. However, this requires overcoming a problem presented by hysteresis characteristic of the stepping motor. For example, in setting the shutter opening angle at 1/500 sec, the direction in which the stepping motor rotates for switching from 1/1000 sec to 1/500 sec is reverse to the direction in which the motor rotates for switching from 1/250 sec to 1/500 sec. Because of the hysteresis characteristic of the stepping motor, the stepping position of the motor varies somewhat depending on its rotating direction. Accordingly, this results in some error in the exposure time. Further, since the exposure time values are arranged in a multiple series, the shutter opening control in the programmed automatic exposure control method must also be performed in a multiple series. Where the shutter opening angle is controlled by a stepping motor through a gear arrangement or the like, and switching from a highest shutter speed time to a next highest shutter speed time is arranged to occur in one step, the response time for the shutter time switch-over becomes unduly long as the number of steps from one shutter time value to another increases in a multiple serial manner.